Optically Transparent Superhydrophobic Thin Film

ABSTRACT

A coating that can be easily applied, clear, well-bonded, and superhydrophobic is disclosed. In one aspect, a method for coating a substrate comprises providing a substrate having a surface, disposing a coating composition adjacent the surface, the composition comprising a hydrophobic fluorinated solvent, a binder comprising a hydrophobic fluorinated polymer, and hydrophobic fumed silica nanoparticles. Also disclosed is an article comprising a coating layer, the coating layer comprising a plurality of nanoparticles partially exposed on an outward surface thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.16/286,545, filed on Feb. 26, 2019, which claims priority to U.S.Provisional Patent Application No. 62/635,993, filed on Feb. 27, 2018.The foregoing applications are incorporated herein by reference.

BACKGROUND

Unless otherwise indicated herein, the materials described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

Superhydrophobic surfaces and coatings having exceptional waterrepellency properties have potential application in numerous fields ofendeavor. Well-bonded, optically clear coatings have been achieved, ashave optically clear, superhydrophobic coatings. But there remains aneed for an easily applied, optically clear, well-bonded,superhydrophobic coating or thin film. This is because the physicalproperties that can achieve these three characteristics tend to bemutually exclusive when using conventional thin film materials andmethods. For example, a superhydrophobic material typically has a micro-to nanometer surface roughness, which tends to scatter light and makesoptical clarity difficult to achieve. Likewise, materials with highoptical clarity tend to have low surface roughness (i.e., a very smoothsurface) and do not usually allow good bonding to low surface energyhydrophobic materials. There further remains a need for superhydrophobiccoatings that can retain hydrophobicity after extensive wear.

SUMMARY

In one aspect, the present disclosure provides a method for coating asubstrate, the method comprising providing a substrate having a surface;disposing a coating composition adjacent the surface; the compositioncomprising a hydrophobic fluorinated solvent, a binder comprising ahydrophobic fluorinated polymer, and silica nanoparticles; andevaporating the fluorinated solvent.

In another aspect, the present disclosure provides an article comprisinga coating layer, the coating layer having an inward surface and anopposing outward surface, the inward surface disposed adjacent asubstrate surface, wherein the coating layer comprises a hydrophobicfluorinated polymer and a plurality of nanoparticles, and at least aportion of the nanoparticles are partially exposed on the outwardsurface of the coating layer.

These as well as other aspects, advantages, and alternatives will becomeapparent to those of ordinary skill in the art by reading the followingdetailed description, with reference where appropriate to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a view of an example superhydrophobic optical thinfilm, including the various thin film layers and associated hydrophobicnanoparticles, according to aspects of the present disclosure.

FIG. 2 illustrates a view of an example superhydrophobic optical thinfilm, including the various thin film layers, associated hydrophobicnanoparticles, and depressions from displaced nanoparticles, accordingto aspects of the present disclosure.

FIG. 3 depicts a flow chart illustrating a method according to exampleembodiments.

DETAILED DESCRIPTION

The following detailed description describes various features andfunctions of the disclosed methods, compositions, and structures. Theillustrative embodiments described herein are not meant to be limiting.It will be readily understood that certain aspects of the disclosedmethods, compositions, and structures can be arranged and combined in awide variety of different configurations, all of which are contemplatedherein.

A superhydrophobic composition that is easily applied and well-bondedwithout sacrificing hydrophobicity or optical transparency is described.“Superhydrophobic,” as used herein, describes surfaces or coatings thathave a water contact angle of at least about 130°. Also as used herein,an “optically transparent” coating transmits at least about 90% ofincident light having a wavelength within the range of 300 nm to 1500nm. “Well-bonded,” as used herein, refers to a composition that, whenapplied as a coating or thin film to a substrate, adheres to thesubstrate so as to not be easily removed with relatively small amountsof shear force (e.g., rubbing) or by exposure to environmentalconditions (e.g., sun, rain, wind, etc.).

In one aspect, the present disclosure provides a composition including

a hydrophobic fluorinated solvent;

a binder comprising a hydrophobic fluorinated polymer;

hydrophobic fumed silica nanoparticles; and

optionally, hydrophobic aerogel nanoparticles.

The hydrophobic fluorinated solvent may be a fluorinated materialcapable of dissolving the binder described herein. To provide goodoptical clarity of the resulting film or coating, it is beneficial thatthe composition include particles that are well-dispersed throughout thedeposition process. Particles that are too large or poorly dispersed canlead to clouding of the superhydrophobic surface. Desirable dispersionmay be achieved by using a suitable hydrophobic fluorinated solvent,which may act as a dispersive agent. In some embodiments, thehydrophobic fluorinated solvent may include a fluorinated alkane,fluorinated trialkylamine, fluorinated cycloalkane, fluorinatedheterocycloalkane, or combination thereof. In some embodiments, thefluorinated component may be perfluorinated. Suitable fluorinatedsolvents are commercially available from a number of sources, such asSigma Aldrich (St. Louis, Mo.), 3M (Maplewood, Minn.), etc. Suitablefluorinated solvents include perfluorooctane, 2H,3H-perfluoropentane,perfluorotributylamine, perfluorodecalin, and perfluorononane, etc., forexample, Fluorinert™ FC-40, Fluorinert™ FC-75, Fluorinert™ FC-770, or anequivalent or similar material.

In some embodiments, the hydrophobic fluorinated solvent may include across-linking silane. The cross-linking silane may be selected fromcross-linking agents known in the art having at least one silicon atom.Suitable cross-linking silanes are commercially available from a numberof sources, such as Sigma Aldrich (St. Louis, Mo.), 3M (Maplewood,Minn.), etc. Suitable cross-linking silanes include, for example,silanes having hydride functionality, vinyl functionality, etc., forexample, Novec™ 2702, Novec™ 2202, Novec™ 1720, or equivalent or similarmaterial. When a cross-linking silane is included, the amount of binderused in the composition may be decreased to about 0.3 wt. % to about 1.0wt. % of the composition.

The fluorinated polymer binder may include a hydrophobic, fluorinatedpolymer that is capable of being dissolved in the hydrophobicfluorinated solvent described herein. The binder may enable thehydrophobic particles to adhere to the surface of a substrate, but ifthe binder is not selected properly or is used in the wrong amount, thebinder may affect the optical clarity of the resulting film or coating.The fluorinated polymer binder is preferably optically clear andamorphous. In some embodiments, the fluorinated polymer binder may be afluroalkyl polymer, fluoroalkoxy polymer, perfluoroalkyl polymer,perfluoroalkoxy polymer, or combination thereof. Suitable fluorinatedpolymer binders are commercially available from a number of sources,such as Solvay (Brussels, Belgium). Suitable fluorinated polymer bindersmay include, for example, Teflon® AF and Hyflon® AD.

The amount of the binder in the composition is related to the ability ofthe composition to form a film or coating with the desiredsuperhydrophobic, optical transparency and well-bonded propertiesdescribed herein. If too much binder is used in the composition, thenanoparticles may be engulfed by the binder to such a degree that thesurface loses its nanotexturing and thus its superhydrophobicproperties. If too little binder is employed, the nanoparticles may notbe effectively bonded to the substrate, and the adherence to thesubstrate may be affected. In some embodiments, the fluorinated polymerbinder is present in about 0.3 wt. % to about 1.5 wt. % of thecomposition. In other embodiments, the binder is present in about 0.8wt. % to about 1.2 wt. % of the composition. The binder may also bepresent in about 0.3 wt. % to about 1.4 wt. %, about 0.4 wt. % to about1.5 wt. %, about 0.3 wt. % to about 1.3 wt. %, about 0.4 wt. % to about1.3 wt. %, about 0.4 wt. % to about 1.2 wt. %, about 0. wt. %5 to about1.2 wt. %, about 0.5 wt. % to about 1.1 wt. %, about 0.5 wt. % to about1.0 wt. %, about 0.6 wt. % to about 1.0 wt. %, about 0.7 wt. % to about1.4 wt. %, about 0.5 wt. % to about 1.5 wt. %, about 0.5 wt. % to about1.2 wt. %, or about 0.3 wt. % to about 0.9 wt. % of the composition.

A variety of fumed silica materials including, for example, fumed silicahaving varying particle size distributions or average particles sizes,or even surface-treated fumed silica, are known in the art. In certainembodiments as otherwise described herein, the fumed silicananoparticles are high surface area, nanostructured and/or nanoporousparticles with an average particle size of about 200 nm or less. Theaverage fumed silica nanoparticle size represents an average lineardimension of the particles (e.g., an average diameter in the case ofsubstantially spherical particles), and it may represent an averagegrain or crystallite size, or, in the case of agglomerated particles, anaverage agglomerate size. In some embodiments, the average fumed silicananoparticle size may be less than about 100 nm, less than about 75 nm,or less than about 50 nm. However, extremely small fumed silicananoparticles (e.g., a few nanometers or less) may be difficult todisperse. In some embodiments, the average fumed silica nanoparticlesize is from about 10 nm to about 200 nm, from about 25 nm to about 100nm or from about 40 nm to about 60 nm.

The hydrophobic fumed silica nanoparticles may be silica nanoparticleschemically modified with a hydrophobic silane. In some embodiments, thenanoparticles are chemically treated with a fluorinated material. Inother embodiments, the nanoparticles are chemically treated withpolydimethylsiloxane (PDMS). Colloidal silicon dioxide made from fumedsilica is prepared by a suitable process to reduce the particle size andmodify the surface properties. The surface properties are modified toproduce fumed silica by production of the silica material underconditions of a vapor-phase hydrolysis at an elevated temperature with asurface modifying silicon compound (e.g., silicon dimethylbichloride).The hydrophobic properties of the fumed silica nanoparticles are aresult of treatment with at least one compound selected from the groupconsisting of organosilanes, fluorinated silanes, and disilazanes.

Suitable organosilanes include, but are not limited toalkylchlorosilanes; alkoxysilanes, e.g., methyltrimethoxysilane,methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane,n-propyltrimethoxysilane, n-propyltriethoxysilane,i-propyltrimethoxysilane, i-propyltriethoxysilane,butyltrimethoxysilane, butyltriethoxysilane, hexyltrimethoxysilane,octyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane,n-octyltriethoxysilane, phenyltriethoxysilane, and polytriethoxysilane;trialkoxyarylsilanes; isooctyltrimethoxy-silane;N-(3-triethoxysilylpropyl)methoxyethoxyethoxy ethyl carbamate;N-(3-triethoxysilylpropyl)methoxyethoxyethoxyethyl carbamate;polydialkylsiloxanes including, e.g., polydimethylsiloxane; arylsilanesincluding, e.g., substituted and unsubstituted arylsilanes; alkylsilanesincluding, e.g., substituted and unsubstituted alkyl silanes including,e.g., methoxy and hydroxy substituted alkyl silanes; and combinationsthereof. Suitable alkylchlorosilanes include, for example,methyltrichlorosilane, dimethyldichlorosilane, trimethylchlorosilane,octylmethyldichlorosilane, octyltrichlorosilane,octadecylmethyldichlorosilane and octadecyltrichlorosilane. Othersuitable materials include, for example, methylmethoxysilanes such asmethyltrimethoxysilane, dimethyldimethoxysilane andtrimethylmethoxysilane; methylethoxysilanes such asmethyltriethoxysilane, dimethyldiethoxysilane and trimethylethoxysilane;methylacetoxysilanes such as methyltriacetoxysilane,dimethyldiacetoxysilane and trimethylacetoxysilane; vinylsilanes such asvinyltrichlorosilane, vinylmethyldichlorosilane,vinyldimethylchlorosilane, vinyltrimethoxysilane,vinylmethyldimethoxysilane, vinyldimethylmethoxysilane,vinyltriethoxysilane, vinylmethyldiethoxysilane andvinyldimethylethoxysilane.

Suitable fluorinated silanes include fluorinated alkyl-, alkoxy-, aryl-and/or alkylaryl-silanes, and fully perfluorinated alkyl-, alkoxy-,aryl- and/or alkylaryl-silanes. An example of a suitable fluorinatedalkoxy-silane is perfluorooctyltrimethoxysilane.

Suitable disilazanes include, for example, hexamethyldisilazane,divinyltetramethyldisilazane andbis(3,3-trifluoropropyl)tetramethyldisilazane. Cyclosilazanes are alsosuitable, and include, for example, octamethylcyclotetrasilazane.

Suitable hydrophobic fumed silica nanoparticles are commerciallyavailable from a number of sources, including Cabot Corporation(Tuscola, Ill.) under the trade name CAB-O-SIL, and Degussa, Inc.(Piscataway, N.J.), under the trade name AEROSIL. Suitable hydrophobicfumed silica particles include, for example, AEROSIL[R]R 202,AEROSIL[R]R 805, AEROSIL[R] R 812, AEROSIL[R]R 812 S, AEROSIL[R] R 972,AEROSIL[R]R 974, AEROSIL[R]R 8200, AEROXIDE [R] LE-1 and AEROXIDE [R]LE-2.

In some embodiments, the hydrophobic fumed silica nanoparticles arepresent in about 0.01 wt. % to about 0.5 wt. % of the composition. Inother embodiments, the hydrophobic fumed silica nanoparticles arepresent in about 0.08 wt. % to about 0.12 wt. % of the composition. Thehydrophobic fumed silica nanoparticles may also be present in about 0.03wt. % to about 0.5 wt. %, about 0.04 wt. % to about 0.5 wt. %, about0.03 wt. % to about 0.4 wt. %, about 0.04 wt. % to about 0.4 wt. %,about 0.04 wt. % to about 0.3 wt. %, about 0.05 wt. % to about 0.2 wt.%, about 0.05 wt. % to about 0.1 wt. %, about 0.05 wt. % to about 0.1wt. %, about 0.06 wt. % to about 0.1 wt. %, about 0.07 wt. % to about0.1 wt. %, about 0.05 wt. % to about 0.5 wt. %, about 0.05 wt. % toabout 0.3 wt. %, or about 0.01 wt. % to about 0.09 wt. % of thecomposition.

In some embodiments, the composition may further include hydrophobicaerogel nanoparticles. The combination of hydrophobic fumed silicananoparticles in conjunction with hydrophobic aerogel nanoparticles mayprovide a coating or film with additional water repellency.Superhydrophobic coatings that include hydrophobic aerogel nanoparticlesbut without fumed silica nanoparticles can provide superhydrophobic,optically clear thin films. But these films fall apart with smallamounts of shear force. Thus, such coatings are easily destroyed byrubbing, and do not provide prolonged protection to the coated surface.A composition including hydrophobic fumed silica nanoparticles, however,provides a more durable superhydrophobic coating, which can be wellbonded to a glass surface. Combining hydrophobic aerogel withhydrophobic fumed silica allows the aerogel to be protected from rubbingshear forces by “hiding” between well-bonded fumed silica nanoparticles(see FIG. 1). The addition of hydrophobic aerogel nanoparticles canfurther increase the film's superhydrophobic behavior while maintaininggood durability.

Suitable hydrophobic aerogel nanoparticles are very high surface area(600-800 m²/g) particles with a density between about 100 and 200 kg/m³and an average particle size of about 200 nm or less. The averageaerogel nanoparticle size represents an average linear dimension of theparticles (e.g., an average diameter in the case of substantiallyspherical particles), and it may represent an average grain orcrystallite size, or, in the case of agglomerated particles, an averageagglomerate size. In some embodiments, the average aerogel nanoparticlesize may be less than about 100 nm, less than about 75 nm, or less thanabout 50 nm. However, extremely small aerogel nanoparticles (e.g., a fewmicrometers or less) may be difficult to disperse. In some embodiments,the average aerogel nanoparticle size is from about 10 nm to about 200nm, from about 25 nm to about 100 nm or from about 40 nm to about 60 nm.

The hydrophobic aerogel nanoparticles may be obtained from a precursorpowder that is processed to reduce the average particle size to about200 nm or smaller. The hydrophobic aerogel nanoparticles may includenanoscale surface asperities, i.e., a nanoscale surface texturecharacterized by protruding or sharp features separated by recessedfeatures and/or pores at the particle surface. Coating compositionsincluding particles with such nanoscale surface asperities may yieldcoatings with higher water contact angles and thus enhancedhydrophobicity. As one of ordinary skill in the art would recognize, thescale of the surface texture is smaller than the average size of theparticle; generally, surface asperities are at least about 50% smaller.For example, aerogel particles of about 100 nm in average particle sizemay include surface asperities of about 25 nm in average size or less,and hydrophobic particles of about 50 nm in average particle size mayinclude surface asperities of about 25 nm in size or less.

Suitable aerogel precursor powders are commercially available from anumber of sources, including Cabot Corp. (Boston, Mass.). Suitableaerogel precursor powders are sold under the Nanogel Aerogel, LUMIRA®Aerogel and ENOVA Aerogel trade names, and include, for example ENOVA™Aerogel IC 3110, ENOVA™ Aerogel MT 1100, ENOVA™ Aerogel MT 1200, ENOVA™Aerogel IC 3120. These porous, nanostructured particles are available inparticle sizes ranging from about 5 micrometers to 4 mm, but may bemechanically milled or sonicated as discussed below to obtain particlesof reduced sizes (e.g., less than about 50 nm) for use in formingsuperhydrophobic coatings.

In another aspect, the present disclosure provides a method for making acomposition as described herein. The method involves:

-   -   (a) combining a hydrophobic fluorinated solvent, a binder        comprising a hydrophobic fluorinated polymer, fumed silica        nanoparticles, and optionally, hydrophobic aerogel        nanoparticles;    -   (b) mixing the combination; and    -   (c) drying the mixture to provide the composition.

In embodiments where the composition includes hydrophobic aerogelnanoparticles, the combination may further include hydrophobic aerogelnanoparticles added prior to mixing. Mixing by sonication, (e.g., with asonic probe) can be used to break up conglomerates of the hydrophobicfumed silica nanoparticles and/or the hydrophobic aerogel nanoparticles,for example, if the conglomerated nanoparticles are large enough toscatter a significant amount of light.

Advantageously, the present inventor has determined such compositionscan be easily applied to a substrate to provide a well-bonded, opticallyclear, hydrophobic coating. Accordingly, another aspect of disclosure isa method for coating a substrate. An example method 300 of coating thesubstrate is illustrated in FIG. 3. At block 302, the method 300 mayinclude providing a substrate having a surface, and at block 304,disposing a coating composition adjacent the surface. In certainembodiments, the method 300 may include at block 308 treating thesubstrate before disposing the coating composition at block 304. Thecomposition comprises a hydrophobic fluorinated solvent, a bindercomprising a hydrophobic fluorinated polymer, and hydrophobic fumedsilica nanoparticles. At block 306, the method may further includeevaporating the fluorinated solvent.

The amounts and identities of the various components can be as otherwisedescribed above with respect to the compositions of the disclosure. Forexample, in certain embodiments as otherwise described herein, thecoating composition further comprises hydrophobic aerogel nanoparticles.

Accordingly, in certain embodiments as otherwise described herein, thebinder is present in the coating composition in an amount within therange of 0.3 wt. % to 1.5 wt %. For example, in certain suchembodiments, the binder is present in an amount within the range of 0.5wt. % to 1.5 wt. %, or 0.8 wt. % to 1.5 wt. %, or 0.3 wt. % to 1.2 wt.%, or 0.8 wt. % to 1.2 wt. % of the coating composition. In certainembodiments as otherwise described herein, the silica nanoparticles arepresent in the coating composition in an amount within the range of 0.01wt. % to 0.5 wt. %. For example, in certain such embodiments, the silicananoparticles are present in an amount within the range of 0.03 wt. % to0.5 wt. %, or 0.05 wt. % to 0.5 wt. %, or 0.08 wt. % to 0.5 wt. %, or0.01 wt. % to 0.4 wt. %, or 0.01 wt. % to 0.25 wt. %, or 0.01 wt. % to0.12 wt. %, or 0.03 wt. % to 0.4 wt. %, or 0.05 wt. % to 0.25 wt. %, or0.08 wt. % to 0.12 wt. % of the coating composition. In certainembodiments as otherwise described herein, the aerogel nanoparticles arepresent in the coating composition in an amount within the range of 0.1wt. % to 0.5 wt. %. For example, in certain such embodiments, theaerogel nanoparticles are present in an amount within the range of 0.2wt. % to 0.5 wt. %, or 0.3 wt. % to 0.5 wt. %, or 0.1 wt. % to 0.4 wt.%, or 0.1 wt. % to 0.3 wt. %, or 0.15 wt. % to 0.45 wt. % of the coatingcomposition.

For example, in certain embodiments as otherwise described herein, theaverage size of the silica nanoparticles, or the average size of thesilica nanoparticles and aerogel nanoparticles, is within the range of10 nm to 200 nm, or 25 nm to 200 nm, or 50 nm to 200 nm, or 100 nm to200 nm, or 10 nm to 150 nm, or 10 nm to 100 nm, or 10 nm to 50 nm, or 25nm to 150 nm, or 50 nm to 100 nm. In some examples, it may be desirableto have the average size of the silica nanoparticles and aerogelnanoparticles equal to 10% or less of the electromagnetic radiationwavelength (i.e., radio waves and/or light), in order to make theparticles transparent to the electromagnetic radiation.

In certain embodiments as otherwise described herein, disposing thecoating composition comprises spraying the composition onto thesubstrate surface. Advantageously, the present inventors have determinedthat unlike other well bonded, superhydrophobic, optically transparentthin films known in the art, a sprayable composition as otherwisedescribed herein can be easily handled and applied. While conventionalcompositions are often applied with complicated, expensive, andcumbersome processes such as physical vapor deposition, the compositiondescribed herein may be applied to the substrate by, for example, spraycoating, spin coating, or dip coating, or by any other depositiontechniques known in the art. Typically, the composition is depositedonto a clear substrate formed of an optically transparent material, suchas glass or acrylic, although other substrates may be used.

In certain embodiments as otherwise described herein, evaporating thefluorinated solvent comprises air drying or heating the substrate and/ordeposited composition at a temperature above the boiling point of thefluorinated solvent. For example, when Fluorinert™ FC-40 (b.p. of 165°C.) is used as the fluorinated solvent, the substrate may be heated to atemperature in excess of 165° C. to promote the evaporation of thefluorinated solvent.

As described above, a cross-linking silane can be included in thecoating composition. In certain such embodiments, the method furthercomprises curing the disposed coating composition. In certainembodiments as otherwise described herein, curing the disposed coatingcomposition comprises heating the disposed composition to a temperaturesufficient to provide a cross-linked coating. For example, in certainsuch embodiments, the method includes disposing a coating compositioncomprising a cross-linking silane adjacent a substrate surface, andcuring the disposed coating composition at a temperature of at least150° C., or at least 175° C., or at least 200° C. for a period of timesufficient to provide a cross-linked coating composition. In certainsuch embodiments, the method includes curing the coating composition fora period of time within the range of 30 min. to 90 min., or 45 min. to90 min., or 60 min. to 90 min, or 30 min. to 75 min., or 30 min. to 60min, or 45 min. to 75 min. For example, in certain embodiments asotherwise described herein, curing the disposed coating compositioncomprises heating the disposed composition to a temperature sufficientto provide a cross-linked coating (e.g., a temperature of at least 150°C.) for about 60 min.

In certain embodiments as otherwise described herein, the methodincludes treating the substrate (for example, at block 308 of method300). For example, in certain such embodiments, treating the substratecomprises depositing a silane on at least a portion of the substratesurface (i.e., before disposing the coating composition). In anotherexample, in certain embodiments as otherwise described herein, treatingthe substrate comprises plasma etching the substrate. In certain suchembodiments, plasma etching the substrate generates hydroxyl functionalgroups on the substrate surface.

Advantageously, the present inventors have determined that treating thesubstrate surface and/or disposing a coating composition including across-linking silane can improve adhesion of the coating composition tothe substrate (e.g., where the substrate is a highly hydrophilicmaterial such as glass). In certain embodiments as otherwise describedherein, a coating composition including a cross-linking silane isdisposed adjacent an untreated substrate surface (e.g., a surfacelacking significant hydroxyl-group functionality and/or silanefunctionality). In other embodiments, a coating composition lacking across-lining silane is disposed adjacent a treated substrate surface(e.g., a plasma-etched and/or silane-functionalized surface). Of coursein certain embodiments as otherwise described herein, a coatingcomposition including a cross-linking silane is disposed adjacent atreated substrate surface (e.g., a plasma-etched surface).

Another aspect of the disclosure is a coated substrate prepared by amethod as described herein. For example, in certain embodiments, thecoated substrate is a structure including a substrate and asuperhydrophobic coating on at least a portion of the substrate. Whenthe coating is on the substrate, the resulting film is superhydrophobic,optically clear and well-bonded to the substrate. The superhydrophobiccoating may have a water contact angle of at least 130°. In in certainsuch embodiments, the superhydrophobic coating has a water contact angleof at least 150°. For example, the water contact angle may be at least130°, at least 135°, at least 140°, at least 145°, at least 150°, atleast 155°, at least 160°, at least 165°, at least 170° or at least175°. In some embodiments, the water contact angle encompasses bothadvancing and receding water contact angles.

In some embodiments, the superhydrophobic coating may have a lighttransmission of at least 95% for wavelengths between 300 nm and 1500 nm,or for visible wavelengths between 400 nm and 700 nm. The substrate mayalso be an optically transparent material such as glass or plastic. Inembodiments where the substrate is also optically transparent, thecoated substrate allows light (e.g., from a laser or optical sensor) tobe transmitted through the substrate and the superhydrophobic coatingwith limited interference. The superhydrophobic nature of the coatingmay also enable the substrate to stay clean and dry by limiting theability for water (e.g., rain) and dirt or dust from accumulating on thesurface.

The superhydrophobic coating may also adhere to the substrate in amanner that does not allow it to be removed by rubbing or by exposure toenvironmental conditions (e.g., sun, rain, wind, etc.). This aspect ofthe superhydrophobic coating allows a single application to remain onthe substrate for a prolonged period of time, and is a characteristicnot previously known for a superhydrophobic, optically transparentcoating.

In some embodiments, the structure further comprises a silane layerdisposed between the superhydrophobic coating and the substrate. Thesilane may be employed to modify the surface energy or wettability ofthe surface of the substrate prior to the application of thesuperhydrophobic composition. The silane may be a silicon-containingcompound having linear alkyl, branched alkyl, or aryl groups, includingdipodal silanes, and may be optionally fluorinated. In some embodiments,the silane is a hydrophobic silane. Suitable silanes include, forexample, organoethoxysilane, trimethoxysilane,(perfluorobutyl)ethyltriethoxysilane,(3,3,3-trifluoropropyl)trimethoxysilane, and any silane described hereinWhen the fluorinated solvent includes a cross-linking silane, however,the fluorinated silane layer may not be necessary.

Another aspect of the disclosure is an article comprising a coatinglayer, the coating layer having an inward surface and an opposingoutward surface, the inward surface disposed adjacent a substratesurface. The coating layer comprises a hydrophobic fluorinated polymerand a plurality of nanoparticles, and at least a portion of thenanoparticles are partially exposed on the outward surface of thecoating layer. In certain embodiments, the coating layer is the driedproduct of a coating composition as otherwise described herein.Accordingly, in such embodiments, the amounts and identities of thevarious components can be as otherwise described above with respect tothe compositions of the disclosure.

For example, in certain embodiments as otherwise described herein, thenanoparticles are selected from one or more of silica nanoparticles andaerogel nanoparticles. In certain such embodiments, the average size ofthe nanoparticles is within the range of 10 nm to 200 nm. For example,in certain embodiments as otherwise described herein, the nanoparticlesare selected from one or more of silica nanoparticles and aerogelnanoparticles, and have an average size within the range of 10 nm to 200nm, or 25 nm to 200 nm, or 50 nm to 200 nm, or 100 nm to 200 nm, or 10nm to 150 nm, or 10 nm to 100 nm, or 10 nm to 50 nm, or 25 nm to 150 nm,or 50 nm to 100 nm.

In certain embodiments as otherwise described herein, the nanoparticlesare dispersed relatively evenly throughout the coating layer. In otherembodiments, the nanoparticles are localized on the outward surface ofthe coating layer (e.g., the product of depositing nanoparticles ontothe surface of the coating layer).

In certain aspects, nanoparticles partially exposed on the outwardsurface of the coating layer can be displaced from the coating layer,for example, due to subjection to weather, providing a correspondingdepression in the outward surface of the coating layer (see FIG. 2). Thepresent inventor has advantageously determined that such depressions,which have an average size on the scale of the nanoparticles of thecoating layer, can provide a hydrophobic nano-textured surface (e.g.,alone or in combination with partially exposed nanoparticles).

Accordingly, in certain embodiments as otherwise described herein, theoutward major surface of the coating layer further comprises a pluralityof depressions. In certain such embodiments, the average size of thedepressions is within the range of 10 nm to 200 nm, or 25 nm to 200 nm,or 50 nm to 200 nm, or 100 nm to 200 nm, or 10 nm to 150 nm, or 10 nm to100 nm, or 10 nm to 50 nm, or 25 nm to 150 nm, or 50 nm to 100 nm.

In certain embodiments as otherwise described herein, the coating layerfurther comprises a cross-linked silane (e.g., the product of curing acoating composition comprising a cross-linking silane). In certainembodiments as otherwise described herein, the substrate comprises aplurality of hydroxyl groups (e.g., the product of plasma etching asubstrate surface). In certain embodiments as otherwise describedherein, the substrate surface comprises a fluorinated silane (e.g., theproduct of depositing a silane onto a substrate surface). In certainembodiments as otherwise described herein, the outward major surface ofthe article has a water contact angle of at least 130°. For example, incertain such embodiments, the outward major surface has a water contactangle of at least 135°, or at least 140°, or at least 150°, or at least155°, or at least 160°.

In certain embodiments, light transmission through the coating layer ofthe article is at least 95% for wavelengths within the range of 300 nmto 1500 nm, or for visible wavelengths within the range of 400 nm to 700nm. In certain embodiments as otherwise described herein, the substratemay also be an optically transparent material, such as, for example,glass or plastic. In certain embodiments, the substrate is opticallytransparent, and the coating layer and substrate allow light (e.g., froma laser or optical sensor) to be transmitted through the substrate andthe coating layer with relatively little interference (e.g., effectivelyno interference).

EXAMPLES Example 1: Formation of a Superhydrophobic Composition

An amorphous fluoropolymer binder is dissolved in a fluorinated solvent.Hydrophobic fumed silica nanoparticles are added. Optionally,hydrophobic aerogel nanoparticles are added. The mixture is mixed with asonic probe to break up conglomerates of the hydrophobic fumed silicaparticles and the hydrophobic aerogel particles, and dried to providethe desired material. Table 1 lists example compositions and the amountsof each component as weight percent of the composition.

TABLE 1 Example Compositions Hydrophobic Hydrophobic Fumed SilicaAerogel Composition Fluorinated Fluoropolymer Nano- Nano- No. SolventBinder particles particles A Fluorinert ™ Hyflon ® AD Aerosil ® ENOVA ®FC-40 (1.0%) (0.5%) Aerogel IC3100 (0.3%) B Fluorinert ™ Hyflon ® ADAerosil ® ENOVA ® FC-40 (0.6%) (0.3%) Aerogel IC3100 (0.2%) CFluorinert ™ Hyflon ® AD Aerosil ® ENOVA ® FC-40 (0.5%) (0.3%) AerogelIC3100 (0.2%) D Fluorinert ™ Hyflon ® AD Aerosil ® ENOVA ® FC-40 (0.3%)(0.15%) Aerogel IC3100 (0%)

Example 2: Superhydrophobic Coating Method

A hydrophobic silane is added to a mixture of a trace amount of waterand isopropyl alcohol or acetone to provide a 1 vol. % silane solution.An optionally plasma etched glass wafer is soaked in the solution, andthen air-dried before heating in an oven at about 100° C. for about15-20 min. to provide a silane-functionalized surface.

A 1-2 wt. % coating solution of an amorphous fluoropolymer binder isprepared in a fluorinated solvent by stirring the binder powder andsolvent at about 50° C. for about 10 min. to provide an optically clear,completely dissolved fluoropolymer solution. The solution is spin-coatedonto the silane-functionalized wafer to provide a 150-450 nm coating.The coated wafer is air-dried before heating in an oven at about 200° C.for about 60 min.

A nanoparticle coating solution including 0.1 wt. % aerogelnanoparticles and 0.2 wt. % silica nanoparticles is prepared in the 1-2wt. % fluoropolymer coating solution by mixing with a sonic probe in 30minute increments until the nanoparticles are sufficiently dispersed.The nanoparticle solution is sprayed onto the coated wafer to provide50-75-nm nanoparticles partially embedded in the coating layer. Thewafer is then air-dried before heating in an oven at about 200° C. forabout 60 min.

The index of refraction of the resulting coating is about 1.33, and thewater contact angle of the coating is about 165°.

Example 3: Superhydrophobic Coating Method

A coating solution of 2 wt. % of fluoropolymer binder and cross-linkingsilane in a fluorinated solvent is spin-coated onto a plasma-etchedglass wafer to provide a 150-450 nm coating. The coated wafer isair-dried before heating in an oven at about 150° C. for about 60 min.to provide a cross-linked coating layer.

A nanoparticle coating solution including 0.1 wt. % aerogelnanoparticles and 0.2 wt. % silica nanoparticles is prepared in the 2wt. % fluoropolymer/cross-linking silane coating solution by mixing witha sonic probe for about 3 hours, until the nanoparticles aresufficiently dispersed. The nanoparticle solution is sprayed onto thecoated wafer to provide nanoparticles partially embedded in coatinglayer. The wafer is air-dried and then heated in an oven at about 150°C. for about 60 min.

The index of refraction of the resulting coating is about 1.41.

Example 4: Superhydrophobic Coating Method

A layer of fluoropolymer binder is deposited onto an optionallyplasma-etched glass wafer using physical vapor deposition (PVD).Following deposition, a nanoparticle coating solution according toExamples 2 or 3 is sprayed onto the coated wafer to providenanoparticles partially embedded in the coating layer. The wafer isair-dried and then heated in an oven at 150-200° C. for 60 min.

Example 5: Extreme Weathering of Coating

The coated wafer of Example 2 is subjected to simulated rainfall andwind for an extended period of time. A portion of the partially embeddednanoparticles are displaced from the coating, resulting in a surfacecomprising the remaining partially embedded nanoparticles andnano-scaled depressions (see FIG. 2). The surface after weatheringremains superhydrophobic, with a water contact angle of about 135°.

It should be understood that arrangements described herein are forpurposes of example only. As such, those skilled in the art willappreciate that other arrangements and other elements can be usedinstead, and some elements may be omitted altogether according to thedesired results.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims, along with the fullscope of equivalents to which such claims are entitled. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting.

1-10: (canceled)
 11. An article comprising a coating layer, the coatinglayer having an inward surface and an opposing outward surface, theinward surface disposed adjacent a substrate surface, wherein thecoating layer comprises a hydrophobic fluorinated polymer and aplurality of nanoparticles, and at least a portion of the nanoparticlesare partially exposed on the outward surface of the coating layer. 12.The article of claim 11, wherein the nanoparticles are selected from oneor more of silica nanoparticles and aerogel nanoparticles.
 13. Thearticle of claim 11, wherein the average size of the nanoparticles iswithin the range of 10 nm to 200 nm.
 14. The article of claim 11,wherein the outward surface further comprises a plurality ofdepressions.
 15. The article of claim 14, wherein the average size ofthe depressions is within the range of 10 nm to 200 nm.
 16. The articleof claim 11, wherein the coating layer further comprises a crosslinkedsilane.
 17. The article of claim 11, wherein the substrate surfacecomprises a plurality of hydroxyl groups.
 18. The article of claim 11,wherein the substrate surface comprises a silane.
 19. The article ofclaim 11, wherein the outward surface has a water contact angle of atleast 130°.
 20. The article of claim 11, wherein the outward surface hasa water contact angle of at least 150°.